From a legal point of view, an S-Corporation is a stand-alone legal entity and protects its shareholder's personal assets. Their financial risk is limited to their investments in the company. From a tax point of view, S-Corporations are "pass-though" entities. They are generating taxable income but do not pay taxes on a corporate level. At the end of the fiscal year, they send each shareholder IRS Form K-1 which passes the tax liability on to each shareholder in proportion to the total funds they invested in the company. (called "shareholders stock basis)
There are three reasons why it is done this way. The IRS wants to prevent that shareholders are deciding to distribute all profits to the member who pays the lowest tax rate and re-distribute the funds amongst each other.
Unless the level of each shareholder's stock and loan basis is up-to-date, reconciled, and signed off regularly, some shareholders could be tempted to "dip into the honey pot" and withdraw funds from the corporation that actually does not belong to them. Both, a trust and a tax issue.
The other is related to the definition of the shareholder's liability and capital at risk in case of litigations.